"In our paper, we introduce a generic and mathematically innovative argument that establishes that an equilibrium black hole must indeed have, as a rule, at least one standard light ring in each rotational sense," Cunha and Herdeiro said. The new theorem they devised provides a solid theoretical basis for the prediction that generic equilibrium black holes must possess at least one light ring orbit. With their study, Cunha and Herdeiro tried to examine the possibility of extrapolating the notion of light ring orbits and applying it to black holes with generic matter content or to alternative theories of gravity (i.e., not the theory of general relativity). Past cosmological observations suggest that these classical black holes possess light ring orbits, which could mean that any conceivable black hole would also have these orbits. Therefore, a key question is: does any black hole model, in any theory of gravity, need to have a light ring?"Īccording to the theory of general relativity, the properties of black holes in a state of equilibrium and in empty space are very constrained. At this moment it is still unclear whether Einstein's theory of general relativity remains a good description of the laws of gravity under such extreme conditions.
"Measuring these properties grants a direct window into the elusive and yet fairly uncharted regime of very strong gravity close to a black hole. "Remarkably, the properties of light rings can encode much relevant black hole information," Pedro Cunha and Carlos Herdeiro, the two researchers who carried out the study, told via email. Their theorem, presented in a paper published in Physical Review Letters, suggests that equilibrium black holes must, as a general rule, have at least one light ring in each of their sense of rotation. Researchers at the Max Planck Institute for Gravitational Physics in Germany and Universidade de Aveiro in Portugal have recently introduced a theorem that makes predictions about the light rings around stationary black holes. So far, however, many questions remain unanswered, and we are still far from gaining a good understanding of both black holes and the light rings surrounding them. Studying these light rings could ultimately enrich our current understanding of black holes and their properties. Nonetheless, they should contain a signature pointing to a neighboring region just outside of the event horizon, wherein light is bent so strongly that its path closes over itself and forms circular orbits known as light rings. While both these observations are highly promising and captivating, neither of them is likely to unveil the event horizon, the boundary defining the specific region in space around a black hole from which nothing can escape. Meanwhile, the Event Horizon Telescope research group captured the very first image of a black hole shadow. The LIGO-Virgo collaboration was able to detect gravitational waves around these celestial objects using some of the most advanced gravitational-wave detectors in the world.
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